This invention relates to an optical prism and particularly to a trichromatic composition prism suitable for formation in an extremely large size for use in a projection TV (television) set, and to a projection TV set using the prism.
One example of a trichromatic resolution optical prism employed in a video camera is as shown in FIG. 3. Such prism is of course much smaller than that of the invention.
In FIG. 3, reference numeral 1 designates a convex lens for converging incident light beams; 2, 3 and 4, color resolving prisms; 5, a first dichroic mirror for transmitting red light and green light, and reflecting blue light; 6, a second dichroic mirror for transmitting green light and reflecting red light; 7 and 8, first and second total reflection surfaces for reflecting light of all colors; 9, an air gap between the first dichroic mirror and the second total reflection surface; 10, a blue image-forming surface; 11, a green image-forming surface; and 12, a red image-forming surface.
The operation of the optical prism thus constructed will be described. Light applied to the convex lens 1 from the left enters the prism 2 through the first total reflection surface 7 and reaches the first dichroic mirror 5, where light beams corresponding in wavelength to a blue component in the spectrum are reflected while the others are transmitted. The blue light reflected by the first dichroic mirror 5 is applied to the first total reflection surface 7 at more than the critical angle, where it is fully reflected to reach the blue image-forming surface 10. The light beams passed through the first dichroic mirror 5 enter the prism 3 and reach the second dichroic mirror 6, where only light corresponding to a red component is reflected. The light beam thus reflected is totally reflected by the second total reflection surface 8, thus reaching the red image-forming surface 12. The light beam passed through the second dichroic mirror 6 enters the prism 4 and reaches the green image-forming surface 1. The gap 9 is an air layer cooperating with the second total reflection surface 8 to permit said total reflection. The optical paths of the light beams in the prisms are made equal in length to one another so that the beams are equal in transmission time.
FIG. 4 shows one example of a conventional trichromatic composition technique for a projection TV set in which dichroic mirrors are so arranged as to form 45.degree. angles with the optical axes. In FIG. 4, reference number 41 designates a compensating lens for gathering and radiating red, blue and green light; 42, a dichroic mirror having a dichroic film which reflects red light and transmits green and blue light; and 43, a dichroic mirror having a dichroic film which reflects blue light and transmits red and green light. The dichroic mirrors 42 and 43 are so arranged as to form 45.degree. angles with the optical axes. The lens 41 is arranged in one of the four spaces defined by the two dichroic mirrors 42 and 43, and convex lenses 44, 45 and 46 and red, green and blue electron guns (not shown) are arranged in the remaining three spaces, respectively.
Red light emitted from the red electron gun passes through the convex lens 44 and is then reflected by the dichroic mirror 43. Blue light emitted from the blue electron gun, after passing through the convex lens 46, is reflected by the dichroic mirror 42. Green light emitted from the green electron gun passes through the dichroic mirrors 42 and 43. Thus, all of the red, blue and green light beams reach the lens 41. Thereafter, these light beams are projected onto a large screen (not shown) through a lens group (not shown) arranged behind the lens 41. As is apparent from the above description, the optical axes of the red, blue and green light beams are coincident with one another after passing through the lens 41. Because of this coincidence of the optical axes, the above-described method is advantageous in permitting electrical adjustments such as convergence adjustment or image distortion adjustment or optical adjustments such as color shift adjustment when compared with a method in which the light beams are allowed to reach the screen through respective lens groups. In addition, for the same reason, the screen can be designed with ease.
In the case where the dichroic mirrors are crossed as described above, an important factor is the transmissivity of the dichroic film which serves as an optical filter. In FIG. 5, the dotted lines indicate the radiation energy distributions of the ordinary blue, green and red electron guns with respect to optical wavelengths, and the solid lines indicate how the transmissivities of the dichroic mirror vary with the incident angle of the blue light beam thereto As is apparent from the graphical representation of FIG. 5, the smaller the incident angle, the more positively the blue light beam can be blocked. In FIG. 4, given the positioning the optical axis of the blue light beam, the incident angle to the dichroic mirror 42 is 45.degree.. If the incident angle is decreased to 25.degree., then the blue light beam can be substantially reflected. If the incident angle is increased to 60.degree., then the blue light beam is passed through; that is, the dichroic mirror is not satisfactory in performance in this case. In FIG. 5, for the incident angle of 60.degree., the characteristic curve is gently wavy. This is due to the interference which is caused owing to the number of layers (for instance fifteen or sixteen layers) forming the dichroic film.
The conventional trichromatic resolution optical prism is constructed as described above. In the case where the prism is made of glass, a high-precision polishing process is required in order to flatten the total reflection surfaces and the dichroic-film-deposition surfaces thereof When it is required to provide a large trichromatic resolution optical prism, for instance, for a projection TV set, the sides of the resultant optical prism will be at least 70 mm long in the case where a 5-inch cathode ray tube is employed, and therefore the optical prism is large in weight, and high both in material cost and in machining costs such as for vacuum deposition and polishing.
On the other hand, dichroic mirror forming techniques using multi-layer vacuum deposition are not yet sufficiently developed. Therefore, when it is required to form an optical prism using plastic, it is considerably difficult to do so, and surface flatness cannot be improved without an additional machining operation.
In the case where, as shown in FIG. 4, the dichroic mirrors cross each other, the incident angle of the light beam to the dichroic mirror should be smaller than 45.degree. even when the light beam is near the center of the mirror. However, as the incident point of the light beam shifts from the center of the mirror, the incident angle is changed, and the light beam reflected by the dichroic film is decreased in intensity Therefore, when the light beam reaches the screen, color shading takes place around the incident point.